This invention relates to a differential circuit for use in obtaining a desirable characteristic.
Heretofore, a wide variety of differential circuits have been proposed so as to accomplish desired characteristics. Herein, such differential circuits are largely classified into a first type of differential circuits formed by bipolar transistors and a second type of differential circuits formed by MOSFET's (Metal Oxide Semiconductor Field Effect Transistors).
A conventional first type of such differential circuits comprises first and second bipolar transistors which have first and second bases, first and second collectors, and first and second emitters, respectively. Both of the first and the second emitters are connected to a constant current circuit in common. With this structure, a differential input voltage is given across the first and the second bases while an output signal is produced in the form of a collector current at either one of the first and the second collectors or between the first and the second collectors.
Herein, it is known that the output signal, namely, the collector current is approximately varied in relation to the differential input voltage in accordance with an exponential law. In addition, it often happens that a hyperbolic sine, cosine, or tangent curve characteristic is required between the differential input voltage and the output signal in order to carry out various kinds of signal conversions. The characteristics between the differential input voltage and the collector current are known as transfer characteristics.
However, the conventional differential circuit of the first type can achieve neither a correct exponential characteristic nor correct hyperbolic sine, cosine, or tangent curve characteristics. This means that an exponential function, a hyperbolic function, a hyperbolic sine function, and a hyperbolic cosine function can not be realized by the use of the differential circuit of the first type, in spite of the fact that such functions are very important for conversions of the signals. Under the circumstances, realization of such various functions is preferable for the signal conversions. Especially, if such functions can be obtained by the use of a differential circuit manufactured by a semiconductor integrated circuit technique, the differential circuit in question can be directly connected to other circuits within a semiconductor chip and can be integrated with the other circuits.
On the other hand, various kinds of second-type differential circuits have been also conventionally proposed which comprise a pair of MOSFETs having sources connected in common to each other. Such a second-type differential circuit is used as a squaring circuit or an operational transconductance amplifier (OTA), because a drain current is varied relative to a voltage between a source and a gate of each of MOSFETs in accordance with a square law. Thus, the second-type differential circuit exhibits a squaring characteristic.
As the differential circuit of the second type, A. Nedungadi and T. R. Viswanathan have proposed a complementary metal-oxide semiconductor (CMOS) OTA in an article entitled "Design of Linear CMOS Transconductance Elements" (IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, Vol. CAS-31, No. 10, pp. 891-894, OCTOBER 1984).
The inventor of the present invention has also proposed a CMOS OTA in a paper described in IEICE TRANSACTIONS ON FUNDAMENTALS, Vol. E75-A, No. 12, pp. 1774-1776, DECEMBER 1992 and entitled "An MOS Operational Transconductance Amplifier and an MOS Four-Quadrant Analog Multiplier Using the Quadritail Cell".
Furthermore, Fran.cedilla.ois Krummenacher and Norbert Joehl have proposed a differential circuit in an article described in IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 23, No. 3, pp. 750-758, JUNE 1988 and entitled "A 4-MHz CMOS Continuous-Time Filter with On-Chip Automatic Tuning".
Moreover, Zhenhua Wang and Walter Guggenbuhl have proposed an OTA in a paper entitled "A Voltage-Controllable Linear MOS Transconductor Using Bias Offset Technique" (ZHENHUA WANG and WALTER GUGGENBUHL, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 25, No. 1, pp. 315-317, FEBRUARY 1990).
In addition, M. F. Li, X. Chen and Y. C. Lim proposed a differential circuit in an article entitled "LINEARITY IMPROVEMENT OF CMOS TRANSCONDUCTORS FOR LOW SUPPLY APPLICATIONS" (ELECTRONICS LETTERS, 10th Jun. 1993, Vol. 29, No. 12, pp. 1106-1107).
At any rate, each of the differential circuits utilizes a squaring characteristic of the MOSFETs. Practically, the squaring circuit or the OTA can be readily structured by the use of a root characteristic of the MOSFET's, as mentioned in a book published by Peter Peregrinus Ltd in London and entitled "High Frequency CMOS Transconductors" (written by Dupuie and Ismail and edited by Toumazou, Lidgey, and Haigh in Analog IC design).
However, the squaring characteristic is determined by a threshold voltage of each of MOSFETs that can not electrically be changed by an external circuit. This shows that the squaring characteristic can not be controlled by a program or the like. Therefore, it is difficult to incorporate the differential circuit of the second type into an integrated circuit together with any other circuit elements.